The present invention relates to a flexible print circuit board (hereinafter referred to as “FPC”) having various surface mountable electronic parts mounted thereon and a method of producing the same and more particularly to a multi-layered FPC having high connection reliability and a method of producing the same.
With the recent trend for smaller size, lighter weight and higher performance of electronic apparatus, the circuit density of FPC to be incorporated therein tends to increase more and more. As a method of increasing the circuit density of FPC, there is used a method involving fine patterning of circuit layer. However, this method is limited in its capability and leaves something to be desired. Under these circumstances, a multi-layered FPC obtained by laminating circuit layers on each other with an adhesive layer interposed therebetween and providing the insulating layer between the circuit layers with an interlayer connection structure to make three-dimensional connection of the circuit layers has bee noted.
Heretofore, such a multi-layered FPC has been arranged such that circuit layers provided on the both sides of an insulating layer are three-dimensionally connected to each other with a copper deposit layer formed on the wall of a through-hole formed in an insulating layer made of polyimide film or the like (see, e.g., JP-A-5-175636). This interlayer connection method is called plated through-hole method and is most usually used. A through-hole method involves two major steps, i.e., step of electrolessly plating the wall of an insulating through-hole to electrically conduct the through-hole and step of electrolytically plating the through-hole to effect thick copper plating. The through-hole method is advantageous in that since the copper deposit layer in the through-hole and the insulating layer in which the through-hole is formed are substantially the same in thermal expansion coefficient, no exfoliation at the connection interface attributed to the difference in thermal expansion coefficient between the copper deposit layer in the through-hole and the insulating layer can occur, giving an excellent connection reliability against heat.
The flexible print circuit board obtained by the aforementioned plated through-hole method is disadvantageous in that thick copper plating causes the rise of the thickness of not only the copper deposit layer on the inner wall of the through-hole but also the copper foil constituting the electrically-conductive layer, making it difficult to finely pattern the conduct pattern of the electrically-conductive layer at the subsequent etching step. Further, the process for interlayer connection involves a number of complicated steps, leaving something to be desired in productivity.
As an interlayer connection method for solving these problems there has been proposed a method which comprises printing a solder paste in the through-hole after the formation of circuit layers, and then fusing and solidifying the solder paste (see, e.g., JP-A-7-176847). This interlayer connection method is advantageous in that as compared with the above proposed plate through-hole method, this method allows interlayer connection by a simple process, making it possible to obtain a high productivity. This interlayer connection method is also advantageous in that since interlayer connection is effected after the formation of circuit layers, the process has no effects on the thickness of the copper foil on the circuit layers, making it unlikely that the fine patterning of circuit layers can be inhibited.
In accordance with this interlayer connection method, however, when solder disposed in the through-hole is heated, it expands thermally beyond the insulating layer because the thermal expansion coefficient of solder is greater than that of the insulating layer. It is thus likely that the difference in thermal expansion coefficient can cause the circuit layer and the solder on the insulating layer to be peeled off each other at the connection interface. Thus, the interlayer connection method using solder leaves something to be desired in connection reliability against heat to disadvantage.
As mentioned above, the interlayer connection in multi-layered FPC by the related art plated through-hole method is excellent in connection reliability but is disadvantageous in fine patterning and producibility of circuit layer. On the other hand, the interlayer connection method using solder allows fine patterning of circuit layer and enhancement of producibility of circuit layer, which can be difficultly attained by the aforementioned through-hole method, but leaves something to be desired in connection reliability.
It has thus been desired in the art of interlayer connection in multi-layered FPC to provide a multi-layered FPC having a high producibility that attains both high connection reliability and fine patterning of circuit layer and its producing method.